Wavelength-dispersive XRF

Prior to impinging on the analyzer crystal, by means of a collimator or sUt, the spread in initial directions of the sample-to-crystal beam is limited. Since the maximum achievable angle on a typical WDXRF spectrometer is around 73°, the maximum wavelength that can be diffracted by a crystal of spacing d is equal to ca. 1.9d. 

The angular dispersion dd/dA of a crystal with spacing 2d is given by  

Crystal Planes 2d (nm) K-line range L-line range 

Among wavelength-dispersive spectrometers, a distinction can be made between single-channel instruments and multi-channel spectrometers. In the former type of instrument, a single dispersive crystal/detector combination is used to sequentially measure the X-ray intensity emitted by a sample at a series of wavelengths when this sample is irradiated with the beam from a high power (2—4 kW) X-ray tube. In a multi-channel spectrometer, many crystal/detector sets are used to measure many X-ray Unes/elements simultaneously. 

---

These samples were measured non-destructively by energy-dispersive XRF with synclirotron radiation excitation (SYXRS), by g-XRF, by wavelength-dispersive XRF (WDXRS), and by Rutherford back scattering (RBS), by X-ray reflectometry (XRR) and by destructive secondary ion mass spectrometry (SIMS) as well (both last methods were used for independant comparison). 

Wavelength-dispersive XRF is generally destructive not so energy-dispersive XRF... 

Figure 5.5 Comparison of EDXRF and WDXRF detection systems. Fluorescent X-rays are emitted by the sample on the left. The upper line shows a wavelength dispersive XRF system the lower shows an energy dispersive system. (Reproduced from Pollard and Heron 1996 44, by permission of the Royal Society of Chemistry.)...

Williams-Thorpe, O., Potts, P. J., and Webb, P. C. (1999). Field-portable non-destructive analysis of lithic archaeological samples by X-ray fluorescence instrumentation using a mercury iodide detector Comparison with wavelength-dispersive XRF and a case study in British stone axe provenancing. Journal of Archaeological Science 26 215-237. 

X-ray fluorescence spectroscopy (XRF) Wavelength-dispersive XRF Is generally destructive not so energy-dispersive XRF Giauque et al. (1993)... 

D2622 Wavelength Dispersive XRF All liquid petroleum products and oils Expensive instrument, yet one of the most widely used methods... 

D4927 Wavelength Dispersive XRF Additives and tube oils Widely used in additives industry... 

D6334 Wavelength Dispersive XRF Gasoline and oxygenate blends Standards must match oxygenated sanqrles... 

D7039 Monochromatic wavelength dispersive XRF Gasoline and diesels New method little field experience... 

Monochromatic WDXRF Traditional wavelength dispersive XRF instrument uses polychromatic excitation. This new technology uses monochromatic focused excitation. Preliminary work has shown a reproducibility of about 2 mg/kg at a level of 10 mg/kg of sulfur in gasolines and diesel fuels. Earlier this year this method was issued as ASTM D 7039. 

X-ray Fluorescence (XRF) is a common technique for sulfur determination in hydrocarbon oils. At-line and laboratory XRF analysis is covered in the ASTM standard test methods for sulfur in petroleum products ASTM Standard Method D 4294 (for Energy Dispersive XRF, EDXRF) [1] and ASTM Standard Method D 2622 (for Wavelength Dispersive XRF, WDXRF) [2]. Polarized EDXRF [3] is also used at-line for the determination of very low sulfur content (<10 ppm sulfur) in diesel and gasoline fuels. 

FIG. 1—Principle of Monochromatic Wavelength Dispersive XRF for sulfur analysis. 

Colorimetric methods, whereby chemical species are determined by their ability to alter the colour intensity of a dye, have limited application in palaeolimnology, because there are more suitable alternative methods for most elements. However, colorimetry remains the method of choice for P (e.g., APHA, 1980). A flow injection method can be used to automate the analysis (e.g.. Mas et al., 1990). If the total P concentration of a sample is required, wavelength dispersive XRF is a good alternative. 

Wavelength-dispersive XRF instramentation is almost exclusively used for (highly reliable and routine) bulk-analysis of materials, e. g., in industrial quality control laboratories. In the field of energy-dispersive XRF instrumentation, next to the equipment suitable for bulk analysis, several important variants have evolved in the last 20 years. Both total reflection XRF (TXRF) and micro-XRF are based on the spatial confinement of the primary X-ray beam so that only a Hmited part of the sample (+ support) is irradiated. This is realized in practice by the use of dedicated X-ray sources. X-ray optics, and irradiation geometries. 

Fig. 11.18 Wavelength-dispersive XRF of a brass sample recorded with a LiF analyzer crystal, showing the characteristic lines of the major elements Cu and Zn and the lines of Cr, Fe, Ni and Pb superimposed on a continuous background (adapted from [12]).

In most analytical procedures, calibration is carried out by means of a calibration curve using com-pound(s) prepared with chemicals of an appropriate purity and verified stoichiometry. Matrix effects must often be taken into account and, consequently, the calibration solutions should be matrix-matched. CRMs of pure compounds may be used for calibration. However, matrix CRMs should in principle not be used for the purpose of calibration unless no other suitable calibrants are available, with the exception of those methods (e.g., spark source mass spectrometry, wavelength-dispersive XRF, etc.) that require calibration with CRMs of a similar, fully characterized matrix (e.g., metal alloys, cements). For such methods, accuracy can only be achieved when certified RMs are used for the calibration. 

WDXRF 8 630 Energy-dispersive XRF (EDXRF) wavelength-dispersive XRF... 

The reason for the lack of RMs is the absence of reliable and accurate methods of analysis. Microanalysis is, hence, in need of at least one method that can be used as a reference tool for other techniques and to link RMs or round robin exercises to the international unit of mass. Micro-XRF can be used for this potentially, especially when used for analyzing microscopic samples, where matrix absorption effects are relatively unimportant. At present, XRF is considered to be a rather poor method for certification purposes due to intense matrix effects resulting from intense radiation absorption and enhancement by secondary fluorescence. In wavelength-dispersive XRF, reliable results can only be obtained through calibration with a set of reference samples of closely similar composition to the unknown sample. In the case of energy-dispersive XRF using monochromatic excitation, the correction for matrix effects is simpler but in this case the method suffers from a number of other drawbacks, such as spectral overlap and poor statistics in the spectra. 

This technique utilizes a highly focussed electron beam (5-30 keV) to probe the elemental composition of the sample. The electrons in the beam interact with the sample over a very minute area and excite the elements present there to produce their characteristic x-rays. These characteristic x-rays are recorded and the elements present in the sample can be identified using a Wavelength Dispersive XRF spectrometer (WDF). Both qualitative and quantitative analysis is possible with this non-destructive method, with ppm detection level and 1% reproducibility. 

X-ray intensity due to a particular element is proportional to the concentration of that element in the sample. There are two types of instrument in production those in which the emitted radiation is separated by wavelength using crystals as gratings, i.e., total reflection XRF (wavelength dispersive XRF WDXRF or total reflection XRF TRXRF) and those in which the radiation is not separated but identified by energy dispersive electronic techniques using solid-state detectors and multi-channel analysers, i.e., energy dispersive XRF (EDXRF). 

XRF spectrometers have two major objectives (a) to determine the spectral distribution of the X-rays emitted from the sample (b) the measurement of the intensity of the selected spectral component. In wavelength-dispersive XRF, the spectrum is dispersed into different wavelengths by Bragg diffraction at different crystals. Intensities are measured by electronic detectors. In energy-dispersive spectrometers both energy dispersion and intensity measurement are performed by electronic detectors thus for... 

---